In general, polyolefin based resins such as polyethylene (PE) and polypropylene (PP) have excellent formability and mechanical properties as well as moisture barrier property, so that these resins have been widely applied in various fields. However, the polyolefin based resin has been avoided in food packing because of its high gas permeability particularly oxygen permeability. To overcome this disadvantage, there was an attempt to increase oxygen barrier property by compounding a nano complex to the polyolefin based resin. However, the attempt did not succeed in improving barrier property and even reduced mechanical properties. Thus, to be used for the food packing film, sheet and container, the polyolefin based resin is produced as a multilayer film by co-extrusion or lamination with ethylene-vinylalcohol copolymer (EVOH), nylon, and polyvinylidenchloride which have excellent oxygen barrier property.
The ethylene-vinylalcohol copolymer is a polymer material having excellent gas barrier property and transparency, so it has been used for packing for food or chemical products. However, the ethylene-vinylalcohol copolymer is limited in use because of high price and comparatively low mechanical properties such as flexibility and stretchability.
To overcome the poor oxygen barrier property of the polyolefin based resin and the poor flexibility and stretchability of the ethylene-vinylalcohol copolymer, the blending of the polyolefin based resin and the ethylene-vinylalcohol copolymer was tried. However, because of the unsatisfactory compatibility between the two, they only produce an irregular mixture which reduces the mechanical properties of the film or sheet products. Therefore, it is an important task to increase regularity of the mixture by selecting an appropriate compatibilizer for the blending.
European Patent No. 0,015,556 (publicized on Sep. 17, 1980) and No. 0,210,725 (publicized on Feb. 4, 1987), and U.S. Pat. No. 4,971,864 (registered on Nov. 20, 1990) describe that a film with improved oxygen barrier property and mechanical properties can be prepared by increasing regularity of the mixture of polyolefin based resin and ethylene-vinylalcohol copolymer by using a compatibilizer which is produced by graft-polymerization of a compound having a polar group with polyethylene. U.S. Pat. No. 5,310,788 describes a method to improve stretchability and flexibility of the mixture of polyolefin based resin and ethylene-vinylalcohol copolymer by using the reactant resulted from the reaction of polyamide (or polyamide oligomer) and polyolefin prepared by graft-polymerization of a compound having a polar group with polyethylene as a compatibilizer.
However, films prepared by the above methods were confirmed to have low oxygen barrier property and poor melting formability because of the large amount of a compatibilizer added to improve compatibility, and if the compatibilizer had been added by a small amount, the transparency and other mechanical properties of the film product would have been in question.
U.S. Pat. No. 5,356,990 (registered on Oct. 18, 1994) describes a method in which ethylene-vinylalcohol copolymer (EVOH) is melted first and then ethylene copolymer is added to improve mechanical properties and product appearance. However, to execute this method, a specially designed extruder or molting apparatus is required.
Korean Patent Application No. 1996-001657 describes a multilayer film prepared by the steps of adding a partially saponificated resin of the ethylene-vinylacetate copolymer, as a compatibilizer, to polyolefin based resin and ethylene-vinylalcohol copolymer respectively; and coating the both sides of the ethylene-vinylalcohol copolymer film with the polyolefin based resin. However, according to this method, the compatibility between the polyolefin based resin and the ethylene-vinylalcohol copolymer is still limited and the production procedure of the multilayer film is troublesome.
It has also been discussed the direct insertion of a polar group to polyolefin based resin in order to improve regularity of the oxygen barrier film. But the conventional methods such as high-temperature high-pressure method and catalytic method have problems of difficulty in production process and low introduction of the polar group.
Therefore, it is required to develop a novel oxygen barrier film and container having excellent mechanical properties and other characteristics such as transparency and oxygen barrier property from the polyolefin resin which has no problem of irregularity and enough polar group introduced.